Atomic force microscopy is based upon the principle of sensing the forces between a sharp stylus or tip and the surface to be investigated. The interatomic forces induce the displacement of the tip mounted on the free end of a cantilever arm.
As described by Binnig et al., "Atomic Force Microscope", Phys. Rev. Lett., Vol. 56, No. 9, Mar. 3, 1986, pp. 930-933, a sharply-pointed tip is attached to a spring-like cantilever beam to scan the profile of a surface to be investigated. The attractive or repulsive forces occurring between the atoms at the apex of the tip and those of the surface result in tiny deflections of the cantilever beam. In its original implementation, a tunneling junction was used to detect the motion of the tip attached to an electrically-conductive cantilever beam. An electrically-conductive tunnel tip is disposed within the tunnel distance from the back of the cantilever beam, and the variations of the tunneling current are indicative of the beam deflection. The forces occurring between the tip and the surface under investigation are determined from the measured beam deflection and the characteristics of the cantilever beam.
The principle of atomic force microscopy has been extended to the measurement of magnetic, electrostatic, and frictional forces, with the tip operating in either contact or near-contact with the surface of the sample. Magnetic force microscopy using a magnetized iron tip is described by Martin et al., "High-resolution Magnetic Imaging of Domains in TbFe by Force Microscopy", Appl. Phys. Lett., Vol. 52, No. 3, Jan. 18, 1988, pp. 244-246. The use of silicon tips coated with a film of magnetic material, such as NiFe or CoPtCr, in magnetic force microscopy is described by Grutter et al., "Magnetic Force Microscopy with Batch-fabricated Force Sensors", J. Appl. Phys., Vol. 69, No. 8, Apr. 15, 1991, pp. 5883-5885. Electrostatic force microscopy is described by Terris et al., "Localized Charge Force Microscopy", J. Vac. Sci. Technol. A, Vol. 8, No. 1, January/February 1990, pp. 374-377. Frictional force microscopy is described in Meyer et al., "Simultaneous Measurement of Lateral and Normal Forces with an Optical-beam-deflection Atomic Force Microscope", Appl. Phys. Lett., Vol. 57, No. 20, Nov. 12, 1990, pp. 2089-2091. As in atomic force microscopy as originally conceived by Binnig et al., the forces in all of these techniques are determined from the measured beam deflection and the characteristics of the cantilever beam. It can be argued that whereas magnetic, van der Waals, electrostatic, and frictional forces differ in magnitude and range of interaction, they are all ultimately atomic in nature. Accordingly, the term "atomic force microscope" as used herein includes any scheme in which a tip attached to a cantilever is moved with respect to a surface, and the deflection of the cantilever is used to ascertain the force exerted on the tip by the sample, regardless of the range or origin of the interaction between the tip and the sample.
In addition to tunneling detection, several other methods of detecting the deflection of the AFM cantilever are available, including optical interferometry, optical beam deflection, capacitive techniques, and more recently piezoresistance. Optical beam deflection is currently the most common form of detection used in commercial instruments.
The principle of piezoresistance to detect the deflection of the AFM cantilever is described in U.S. Pat. No. 5,345,815. The cantilever is formed of single-crystal silicon which is implanted with a dopant to provide a piezoresistive region along the length of the cantilever. Deflection of the free end of the cantilever produces stress in the cantilever. That stress changes the electrical resistance of the piezoresistive region in proportion to the cantilever's deflection. A resistance measuring apparatus is coupled to the piezoresistive region to measure its resistance and to generate a signal corresponding to the cantilever's deflection. Moving a cantilever across a sample for scanning is relatively straightforward with piezoresistive detection in comparison to optical detection, for which external optics must move with the cantilever.
AFM systems have applications beyond their original application of imaging the surface of a sample.
AFM systems have been proposed for data storage, as described in U.S. Pat. No. 5,537,372. In that application, the cantilever tip is in physical contact with the surface of a data storage medium. The medium has surface incongruencies in the form of bumps and/or depressions that represent data. The deflection of the cantilever is detected and decoded to read the data. Data can also be written on the medium, if the medium has a heat-deformable surface, by heating the cantilever tip when it is in contact with the medium surface to form bumps or depressions on the medium surface. The tip is heated by a laser beam directed to the tip region of the cantilever. In another approach for heating the tip, as described in Chui et al., "Improved Cantilevers for AFM Thermomechanical Data Storage", Proceedings of Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 2-6, 1996, pp. 219-224, a single-crystal silicon cantilever is selectively doped with boron to provide a conductive path to an electrically-resistive region near the cantilever tip. The tip is then resistively heated when current is passed through the conductive path.
AFM systems have also been proposed for direct writing or lithographically patterning the surface of a semiconductor sample. This technique is referred to as AFM-based lithography or scanning probe lithography (SPL). One type of SPL is described in Minne et al., "Fabrication of 0.1 .mu.m Metal Oxide Semiconductor Field-effect Transistors with the Atomic Force Microscope", Appl. Phys. Lett., Vol. 66, No. 6, Feb. 6, 1995, pp. 703-705; and Minne et al., "Atomic Force Microscope Lithography Using Amorphous Silicon as a Resist and Advances in Parallel Operation", J. Vac. Sci. Technol. B, Vol. 13, No. 3, May/June 1995, pp. 1380-1385. In this type of SPL, the cantilever tip is in contact or near-contact with the semiconductor surface and an electrical potential can be applied between the tip and the semiconductor. The electric field between the tip and the semiconductor surface causes local oxidation on the surface of the semiconductor. As the tip is scanned across the surface and electric potential is cycled on and off in a controlled manner, insulative lines of the oxide are patterned on the semiconductor, which can then be used as a mask for further processing. In another form of SPL, described in Majumdar et al., "Nanometer-scale Lithography Using the Atomic Force Microscope", Appl. Phys. Lett., Vol. 61, No. 19, Nov. 9, 1992, pp. 2293-2295, electrical current from a gold-coated AFM tip has been used to chemically modify a thin layer of the electron beam resist PMMA. After such exposure, a developing step is used to remove either the exposed or unexposed region, leaving a lithographic pattern of resist which can be used as a mask for further processing. Other types of SPL also involve modifying the surface of the semiconductor substrate through the use of a resist layer. For example, a technique of using an AFM tip to plow through the first of two layers of resist and then performing a development step is described in Sohn et al., "Fabrication of Nanostructures Using Atomic-force-microscope-based Lithography", Appl. Phys. Lett., Vol. 67, No. 11, Sep. 11, 1995, pp. 1552-1554.
In prior art AFM systems, the cantilever tip is formed on the end of the cantilever to extend out of the plane of the cantilever in a direction generally perpendicular to the length of the cantilever. Thus, during scanning, the cantilever is oriented generally parallel to the sample and the tip extends downward perpendicularly toward the surface of the sample. This perpendicular out-of-plane tip is formed either as a separate structure added to the cantilever end, as shown in U.S. Pat. No. 5,357,787, or as an integral part of the cantilever, as shown in U.S. Pat. Nos. 5,021,364; 5,051,379; and 5,444,244.
It is desirable to make AFM cantilevers relatively thin. This reduces the mass of the cantilever, thereby allowing the AFM to operate at a higher frequency. Also, in the case of piezoresistive cantilevers for a fixed stiffness, the deflection sensitivity of the cantilever is inversely proportional to its thickness. However, it is difficult to combine the prior art integral out-of-plane perpendicular tips with thin, single-crystal silicon cantilevers. The integral tip is preferable, as it is mechanically more robust than a tip which is added in some way, either through gluing or deposition, as discussed in U.S. Pat. No. 5,357,787; and in Wendel et al., "Sharpened Electron Beam Deposited Tips for High Resolution Atomic Force Microscope Lithography and Imaging", Appl. Phys. Lett., Vol. 67, No. 25, Dec. 18, 1995, pp. 3732-3734. In the particular case of an integral tip extending perpendicularly from the silicon cantilever, a significant amount of material must be removed, typically by an etching process. This tip-formation process is described in U.S. Pat. No. 5,444,244. The etching process makes it difficult to control the final thickness of the cantilever, and thus the desired stiffness, which is a function of the cube of thickness. It is also difficult to make out-of-plane tips out of single-crystal silicon with precisely-controlled geometries, as process variations can alter the final shape of the tip. Different shaped tips will in general have different resolving power, will withstand different amounts of stress in directions both parallel and perpendicular to the cantilever axis, and will also give rise to different degrees of wear. A controllable tip geometry allows the tip shape to be optimized for a particular application.
What is needed is an AFM system with a thin piezoresistive cantilever that can operate at high frequencies, has a cantilever tip integral with the cantilever arm, and can be easily manufactured with a controllable tip geometry.